This application claims the priority of German patent document 199 34 162. 1, filed 21 Jul. 1999 and PCT International Application No. PCT/DE00/02220, filed 6 Jul. 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus system for producing video screen holograms, and to a video screen hologram.
Video screen holograms, which are produced as holographic images of real white video screens or video projection screens and are recorded by means of lasers in the primary colors red, green and blue (rgb), have the advantage that they are effective only within a narrow spectral range about the recording wavelength, and at the same time only within a very limited projection angle about the direction of incidence of the reference beam during the preceding hologram recording. The method of operation of such screens or holographic video screens is described in detail in German Patent Applications No. 19700162.9 and No. 19703592.2.
As the light sources for the projection, rgb lasers in the continuous wave mode and in the pulsed mode as well as rgb light-emitting diodes can mainly be used. The image build-up can optionally be implemented by the serial scanning of a collimated laser beam or by the imaging of an image modulator in the expanded beam of a laser or a light-emitting diode onto the screen. Holographic video screens can be produced for both front and rear projection. Because of their wavelength selectivity and directional selectivity, bright, high-contrast images of a high color fidelity can be projected onto video screen holograms even in a daylight environment. The projector with the narrowband light sources is placed at the beam origin of the divergent reference beam. Only from there will the projection light be efficiently diffracted out of the hologram toward the viewer. The diffuse broadband ambient light, for example, can traverse the video screen hologram unhindered from all other directions of incidence.
U.S. Pat. No. 4,500,163 discloses, for example, a holographic projection screen which has an arrangement of partial screen surfaces. Each partial screen surface is formed by a hologram, during whose recording a diffuser is holographically imaged as an object by means of an objective beam and a reference beam in a photo plate. This is to permit a speckle-free imaging for large-surface, semicircular projection screens.
U.S. Pat. No. 5,926,294 also shows the production of a hologram element in which a diffuser plate is holographically imaged as an objective. The hologram elements are assembled to form a projection screen.
For the reproduction or the projection of images onto video screen holograms, for example, rgb lasers are used, as described in “RBG Optical Parametric Oscillator Source”, K. Snell et al., Aerosense 99, and in German Patent Documents DE 195 04 047 and DE 44 32 029.
The possible applications of video screen holograms extend over the broad field of small displays (for example, for a single person in vehicles and airplanes or at office workstations), to large-surface screens for several viewers at events. Smaller displays can be recorded by means of frequency-stable rgb continuous-wave lasers. In this case, the high requirements with respect to the stability of the laser and of the course of the beam and the connected high expenditures are disadvantageous and result in high cost.
An image representation on large video screen holograms which is independent of the ambient light would be very attractive, for example, for many different applications in the home and the office, for television, computers and electronic cinema and for projections in lecture halls, movie theaters and open-air theaters. However, the production of larger video screens, for example, of the size of typewriter paper (DIN A4) or larger, presents considerable technical difficulties.
First, the output power of the strongest continuous-wave lasers for hologram recordings is currently limited to only a few watts, which, for the particularly suitable silver halide and photo polymer materials, starting at a size of 1 m2, requires a lumination time of tens of minutes. For these long lumination times, the requirements for the mechanical and thermal stability of the material, with respect to the optical components of the beam path and with respect to the frequency stability of the laser, are particularly high.
Second, during recording of the hologram, the object beam and reference beam must be expanded at the expense of the light output over the size of the video screen and of the hologram, because the radial intensity distribution is not constant over the laser beam. Rather, it follows a normal distribution which requires a considerable widening in order to achieve a lighting which is as homogeneous as possible over the surface of the holograms. When several video screen holograms are joined together to create a large video screen hologram, the intensity drop toward the edges of the individual luminated holograms is particularly disturbing because a periodic shadow pattern then runs through the entire video screen during the projection.
Third, the luminating of the three rgb colors of three lasers into the same hologram results in difficulties because a uniform luminating of all three lasers over a larger surface is difficult to achieve. Because the wavelengths of the lasers are clearly different, differences occur in the beam transmission by refraction, diffraction and scattering at different points of the entire beam path, which leads to a nonuniform color display and is difficult to eliminate in the case of an imaging over an extensive surface.
It is therefore an object of the present invention to provide a method and apparatus which can produce high-quality small and large-surface video screen holograms in a simple manner.
Another object of the invention is to create a video screen hologram which can also be implemented on a large-surface without degradation of the image quality during projection.
These and other objects and advantages are achieved by the method and apparatus for producing video screen holograms according to the invention, in which a real video screen is illuminated by means of a narrowband light to produce a hologram of the real video screen, and a large number of individual recordings are made in which only one partial area of the real video screen is illuminated. In this manner, a video screen hologram of the entire video screen is obtained by a composition and/or superimposing of the individual recordings. Because the illumination is performed by means of a scanning pulsed laser beam, it is possible to use only very brief luminating times so that disturbances during the lumination (for example, by shocks or other instabilities) are avoided. Furthermore, there is no occurrence of intensity reductions at the edge or of periodic shadow patterns, and a uniform color display becomes possible on large surfaces.
The pulse duration is, for example, dimensioned such that the movement of the laser beam over the video screen has no influence on the interference of the light waves in the hologram. The recorded partial areas of the video screen preferably correspond to the size of image pixels or are larger. In particular, the lumination can take place by means of a pulsed diode-pumped solid-state continuous-wave laser.
A frequency conversion preferably takes place in one or more of the wavelength ranges red, green, blue. For example, a contact hologram or a video screen plane hologram is produced. A transmission hologram or a reflection hologram may also be produced. Laser beams are preferably generated with a coherence length which is greater than the difference of the light paths between the object beam and the reference beam. The scanning rate and the pulse duration are, for example, coordinated with one another such that the movement of the laser beam during a pulse is less than 1/10 of the wavelength.
Preferably, the video screen surface is scanned repeatedly by means of a respectively phase-shifted laser beam. The distribution of the lumination can be measured in order to correct the lumination during a subsequent lumination cycle. Several luminations may also be carried out by means of light rays or laser beams which are polarized perpendicular to one another in order to produce two mutually independent screen images in the hologram. Furthermore, several luminations with changed recording parameters can be carried out, for example, a changed site of the real video screen or a changed place of origin of the reference beam. The lumination preferably takes place simultaneously by means of light rays or laser beams of the primary colors red, green, blue, which are coaxially adjusted on a beam axis.
The device according to the invention for producing video screen holograms has a narrow-band light source for illuminating a real video screen. This light source is, for example, arranged such that the light emanating from the video screen is superimposed with a reference beam in order to produce a hologram of the video screen, in which case, a scanning device is provided for guiding the light radiation emanating from the light source over the video screen, the light source generating a pulsed light radiation. As a result, high-quality large-surface video screen holograms can also be produced in a simple manner.
The light source preferably simultaneously generates a red, green and blue laser radiation. The light source comprises particularly a laser system which will be described in the following.
The laser system used for producing RGB rays is suitable particularly for the generating of video screen holograms. It comprises a laser beam source for generating laser radiation, a frequency conversion device, and an optical-parametric oscillator, the laser beam source comprising a pulsed q-switched laser oscillator. By means of this laser system, large-surface video screen holograms for a color projection with a high image quality can be produced in a simple manner.
The q-switched laser oscillator is preferably a single-frequency IR oscillator. The laser beam source has, for example, a laser amplifier which is connected behind the q-switched laser oscillator. The video screen hologram according to the invention has a holographic recording material in which a real video screen is stored as a hologram, and contains a large number of individual recordings in each of which a partial area of the real video screen is imaged as a hologram, the entire image of the video screen resulting from assembled and/or superimposed individual recordings. This results in a high-quality image reproduction, even in the case of a large-surface implementation of the video screen hologram.
The video screen hologram is preferably produced according to the method of the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.